In our local Galactic neighborhood , molecular clouds are best studied using a combination of dust measurements , to determine robust masses , sizes and internal structures of the clouds , and molecular-line observations to determine cloud kinematics and chemistry .
We present here the first results of a program designed to extend such studies to nearby galaxies beyond the Magellanic Clouds .
Utilizing the wideband upgrade of the Submillimeter Array ( SMA ) at 230 GHz we have obtained the first continuum detections of the thermal dust emission on sub-GMC scales ( \sim 15 pc ) within the Andromeda galaxy ( M 31 ) .
These include the first resolved continuum detections of dust emission from individual GMCs beyond the Magellanic Clouds .
Utilizing a powerful capability of the SMA , we simultaneously recorded CO ( 2-1 ) emission with identical ( u, v ) coverage , astrometry and calibration , enabling the first measurements of the CO conversion factor , \alpha _ { CO ( 2 - 1 ) } , toward individual GMCs across an external galaxy .
Our direct measurement yields an average CO–to–dust mass conversion factor of \alpha ^ { \prime } _ { CO - dust } = 0.042 \pm 0.018 M _ { \odot } ( K km s ^ { -1 } pc ^ { 2 } ) ^ { -1 } for the J = 2 - 1 transition .
This value does not appear to vary with galactocentric radius .
Assuming a constant gas-to-dust ratio of 136 , the resulting \alpha _ { CO } = 5.7 \pm 2.4 M _ { \odot } ( K km s ^ { -1 } pc ^ { 2 } ) ^ { -1 } for the 2-1 transition is in excellent agreement with that of Milky Way GMCs , given the uncertainties .
Finally , using the same analysis techniques , we compare our results with observations of the local Orion molecular clouds , placed at the distance of M 31 and simulated to appear as they would if observed by the SMA .